It has been well known that the reaction of couplers with aromatic primary amine type color developing agents which are oxidized in advance using optically exposed silver halides as an oxidizing agent can afford indophenols, indoanilines, indamines, azomethines, phenoxazines, phenazines and dyes similar thereto and therethrough color images can be formed.
Now, let us concentrate our discussion on magenta color images. In order to form the magenta color image, couplers of 5-pyrazolone type, cyanoacetophenone type, indazolone type, pyrazolobenzimidazole type, and pyrazolotriazole type can be employed.
It is 5-pyrazolones that occupy most of magenta color image-forming couplers which have been prevailingly submitted to practical use heretofore, and of which wider and deeper studies have been made. Dyes formed from couplers of 5-pyrazolone type are excellent in fastness to heat and light, but they are known to have, in the vicinity of 430 nm, unnecessary absorption of light corresponding to the yellow component which causes turbidity to the resultant color.
For the purpose of reducing the above-described yellow component, a pyrazolobenzimidazole nucleus described in British Pat. No. 1,047,612; an indazolone nucleus described in U.S. Pat. No. 3,770,447; and a pyrazolotriazole nucleus described in U.S. Pat. No. 3,725,067 have been proposed as nuclear skeletons of couplers for forming magenta color images.
However, the magenta couplers described in the above-described patents leave something to be desired, since when mixed with silver halide emulsions in such a state that they are dispersed in hydrophilic protective colloids like gelatin, some of them provide unsatisfactory color images; some of them have low solubilities to high boiling point organic solvents; some of them are difficult of syntheses; and others can only exhibit comparatively low coupling activities when processed with usual developing solutions.